Mocking static methods and the Gateway pattern

This post was originally posted here.

A year ago I started to use mocking libraries (e.g., Mockito, EasyMock, …), both for learning something new and for testing purpose in hopeless cases.
Briefly: such a library makes it possible to dynamically redefine the behaviour (return value, thrown exceptions) of the methods of the class under test, in order to run tests in a controlled environment. It makes it possible even to check behavioural expectations for mock objects, in order to test the Class Under Test’s interactions with its collaborators.
A few weeks ago a colleague asked me: “[How] can I mock a static method, maybe using a mock library?”.
In detail, he was looking for a way to test a class whose code was using a static CustomerLoginFacade.login(String username, String password) method provided by an external API (an authentication custom API by a customer enterprise).
His code looked as follows:

public class ClassUnderTest {
  ...
 public void methodUnderTest(...) {
  ...
   // check authentication
   if(CustomerLoginFacade.login(...)) {
      ...
  } else {
    ...
   }
 }
}

but customer’s authentication provider was not accessible from test environment: so the main (but not the only: test isolation, performances, …) reason to mock the static login method.

A quick search in the magic mocking libraries world revealed that:

• EasyMock supports static methods mocking using extensions (e.g, Class Extension, PowerMock)

• JMock doesn’t support static method mocking

• Mockito (my preferred [Java] mocking library at the moment) doesn’t support static method mocking, because Mockito prefers object orientation and dependency injection over static, procedural code that is hard to understand & change (see official FAQ). The same position appears even in a JMock-related discussion. PowerMock provides a Mockito extension that supports static methods mocking.
  So, thanks to my colleague, I will analize the more general question “Ho can I handle external / legacy API (e.g., static methods acting as service facade) for testing purposes?”. I can identify three different approaches:

• mocking by library: we can use a mocking library supporting external / legacy API mocking (e.g, class’ mocking, static methods’ mocking), as discussed earlier

• mocking by language: we can refer to the features of a dynamically typed programming language to dynamically change external / legacy API implementation / behaviour. E.g., the login problem discussed earlier can be solved in Groovy style, using the features of a language fully integrated with the Java runtime:

  CustomerLoginFacade.metaClass.'static'.login = {
                return true;
  }; 

  Such an approach can be successfully used when CustomerLoginFacade.login‘s client code is Groovy code, not for old Java client code.

• Architectural approach: mocking by design. This approach refers to a general principle: hide every external (concrete) API behind an interface (i.e.: coding on interfaces, not on concrete implementation). This principle is commonly knows as dependency inversion principle.
  So, we can solve my colleague’s problem this way: first, we define a login interface:

  public interface MyLoginService {
   public abstract boolean login(final String username, final String password);
  }

  Then, we refactor the original methodUnderTest code to use the interface:

public class ClassUnderTest {
  private MyLoginService loginService;
 // Collaborator provided by Constructor injection (see here for
 //  a discussion about injection styles)
 public ClassUnderTest(final LoginService loginService) {
  this.loginService = loginService;
 }   
 ...
 public void methodUnderTest(...) {
  ...
   // check authentication
   if(loginService.login(...)) {
      ...
  } else {
    ...
   }
 }
}

So, for testing pourposes, we can simply inject a fake implementation of the MyLoginService interface:

public void myTest() {
 final ClassUnderTest cut = new ClassUnderTest(new FakeLoginService());
 cut.methodUnderTest(..., ...);
 ...
}

where FakeLoginService is simply

public class FakeLoginService implements MyLoginService {
 public boolean login(final String username, final String password) {
  return true;
 }
}
and the real, pruduction implementation of the interface looks simply like this:

public class RealLoginService implements MyLoginService {
 public boolean login(final String username, final String password) {
  return CustomerLoginFacade.login(username, password);
 }
}

Ultimately, the interface defines an abstract gateway to the external authentication API: changing the gateway implementation, we can set up a testing environment fully decoupled from real customer’ authentication provider
.
IMHO, i prefer the last mocking approach: it’s more object oriented, and after all… my colleague called me once the more OO person I know :-). I find this approach more clean and elegant: it’s built only upon common features of programming languages and doesn’t refer to external libraries nor testing-oriented dynamic languafe features.
In terms of design, too, I think it’s a more readable and more reusable solution to the problem, which allows a clearer identification of responsibilities of the various pieces of code: MyLoginService defines an interface, and every implementation represents a way to implement it (a real-life (i.e.: production) implementation versus the fake one).

However, method mocking (by library or by language, doesn’t matter) is in certain, specific situations a very useful technique, too, especially when code that suffers static dependencies (ClassUnderTest in our example) is an example of legacy code, designed with no testing in mind, and is eventually out of developer control.
[Incidentally: the solution adopted by my colleague was just that I have proposed (i.e., mocking by design)]

Credits: thanks to Samuele for giving me cause to analyze such a problem (and for our frequent and ever interesting design-related discussion).
Thanks to my wife for hers valuable support in writing in my pseudo-English.